Chemical vapor deposition involves directing one or more gases containing chemical species onto a surface of a substrate so that the reactive species react and form a deposit on the surface. For example, compound semiconductors can be formed by epitaxial growth of a semiconductor material on a substrate. The substrate typically is a crystalline material in the form of a disc, commonly referred to as a “wafer.” Compound semiconductors such as III-V semiconductors commonly are formed by growing layers of the compound semiconductor on a wafer using a source of a Group III metal and a source of a group V element. In one process, sometimes referred to as a “chloride” process, the Group III metal is provided as a volatile halide of the metal, most commonly a chlorides such as GaCl2 whereas the Group V element is provided as a hydride of the Group V element. In another process, commonly referred to as metal organic chemical vapor deposition or “MOCVD” the chemical species include one or more metal organic compounds such as alkyls of the Group III metals gallium, indium, and aluminum, and also include a source of a Group V element such as one or more of the hydrides of one or more of the Group V elements, such as NH3, AsH3, PH3 and hydrides of antimony. In these processes, the gases are reacted with one another at the surface of a wafer, such as a wafer of sapphire, Si, GaAs, InP, InAs or GaP, to form a III-V compound of the general formula InxGayAlzNAAsBPCSbD where X+Y+Z=approximately 1, and A+B+C+D=approximately 1, and each of X, Y, Z, A, B, C and D can be between 0 and 1. In some instances, bismuth may be used in place of some or all of the other Group III metals.
In either process, the wafer is maintained at an elevated temperature within a reaction chamber. The reactive gases, typically in admixture with inert carrier gases, are directed into the reaction chamber. Typically, the gases are at a relatively low temperature, as for example, about 50-60° C. or below, when they are introduced into the reaction chamber. As the gases reach the hot wafer, their temperature, and hence their available energy for reaction, increases.
One form of apparatus which has been widely employed in chemical vapor deposition includes a disc-like wafer carrier mounted within the reaction chamber for rotation about a vertical axis. The wafers are held in the carrier so that surfaces of the wafers face upwardly within the chamber. While the carrier is rotated about the axis, the reaction gases are introduced into the chamber from a flow inlet element above the carrier. The flowing gases pass downwardly toward the carrier and wafers, desirably in a laminar plug flow. As the gases approach the rotating carrier, viscous drag impels them into rotation around the axis, so that in a boundary region near the surface of the carrier, the gases flow around the axis and outwardly toward the periphery of the carrier. As the gases flow over the outer edge of the carrier, they flow downwardly toward exhaust ports disposed below the carrier. Most commonly, this process is performed with a succession of different gas compositions and, in some cases, different wafer temperatures, to deposit plural layers of semiconductor having differing compositions as required to form a desired semiconductor device. Merely by way of example, in formation of light emitting diodes (“LEDs”) and diode lasers, a multiple quantum well (“MQW”) structure can be formed by depositing layers of III-V semiconductor with different proportions of Ga and In. Each layer may be on the order of tens of Angstroms thick, i.e., a few atomic layers.
Apparatus of this type can provide a stable and orderly flow of reactive gases over the surface of the carrier and over the surface of the wafer, so that all of the wafers on the carrier, and all regions of each wafer, are exposed to substantially uniform conditions. This, in turn promotes uniform deposition of materials on the wafers. Such uniformity is important because even minor differences in the composition and thickness of the layers of material deposited on a wafer can influence the properties of the resulting devices.
The wafer temperature normally is set to optimize the desired deposition reaction; it is commonly above 400° C. and most typically about 700°-1100° C. It is generally desirable to operate equipment of this type at the highest chamber pressure, lowest rotation speed and lowest gas flow rate which can provide acceptable conditions. Pressures on the order of 10 to 1000 Torr, and most commonly about 100 to about 750 Torr, are commonly used. Lower flow rates are desirable to minimize waste of the expensive, high-purity reactants and also minimize the need for waste gas treatment. Lower rotation speeds minimize effects such as centrifugal forces and vibration on the wafers. Moreover, there is normally a direct relationship between rotation speed and flow rate; under given pressure and wafer temperature conditions, the flow rate required to maintain stable, orderly flow and uniform reaction conditions increases with rotation rate.
Prior to the present invention, however, the operating conditions which could be used were significantly constrained. It would be desirable to permit lower rotation speeds and gas flows, higher operating pressures, or both, while still preserving the stable flow pattern.